Phosphatidylinositol (hereinafter abbreviated as “PI”) is one of the phospholipids in cell membranes. In recent years it has become clear that PI plays an important role also in intracellular signal transduction. It is well recognized in the art that PI (4,5) bisphosphate (PI(4,5)P2 or PIP2) is degraded into diacylglycerol and inositol (1,4,5) triphosphate by phospholipase C to induce activation of protein kinase C and intracellular calcium mobilization, respectively [M. J. Berridge et al., Nature, 312, 315 (1984); Y. Nishizuka, Science, 225, 1365 (1984)].
Phosphatidylinositol-3 kinase (“PI3K”) is an enzyme that phosphorylates the 3-position of the inositol ring of phosphatidylinositol [D. Whitman et al., Nature, 332, 664 (1988)]. Pluralities of PI3K subtypes exist. Three major subtypes of PI3Ks have now been identified on the basis of their in vitro substrate specificity, and these three are designated class I (a & b), class II, and class III [B. Vanhaesebroeck, Trend in Biol. Sci., 22, 267 (1997)].
The class Ia PI3K subtype has been most extensively investigated to date. Within the class Ia subtype there are three isoforms (α, β, & δ) that exist as hetero dimers of a catalytic 110-kDa subunit and regulatory subunits of 50-85 kDa. The regulatory subunits contain SH2 domains that bind to phosphorylated tyrosine residues within growth factor receptors or adaptor molecules and thereby localize PI3K to the inner cell membrane. At the inner cell membrane PI3K converts PIP2 to PIP3 (phosphatidylinositol-3,4,5-trisphosphate) that serves to localize the downstream effectors PDK1 and Akt to the inner cell membrane where Akt activation occurs. Activated Akt mediates a diverse array of effects including inhibition of apoptosis, cell cycle progression, response to insulin signaling, and cell proliferation. Class Ia PI3K subtypes also contain Ras binding domains (RBD) that allow association with activated Ras providing another mechanism for PI3K membrane localization. Activated, oncogenic forms of growth factor receptors, Ras, and even PI3K kinase have been shown to aberrantly elevate signaling in the PI3K/Akt/mTOR pathway resulting in cell transformation. As a central component of the PI3K/Akt/mTOR signaling pathway PI3K (particularly the class Ia a isoform) has become a major therapeutic target in cancer drug discovery.
Substrates for class I PI3Ks are PI, PI(4)P and PI(4,5)P2, with PI(4,5)P2 being the most favored. Class I PI3Ks are further divided into two groups, class Ia and class Ib, because of their activation mechanism and associated regulatory subunits. The class Ib PI3K is p110γ that is activated by interaction with G protein-coupled receptors. Interaction between p110γ and G protein-coupled receptors is mediated by regulatory subunits of 110, 87, and 84 kDa.
PI and PI(4)P are the known substrates for class II PI3Ks; PI(4,5)P2 is not a substrate for the enzymes of this class. Class II PI3Ks include PI3K C2α, C2β and C2γ isoforms, which contain C2 domains at the C terminus, implying that their activity is regulated by calcium ions.
The substrate for class III PI3Ks is PI only. A mechanism for activation of the class III PI3Ks has not been clarified. Because each subtype has its own mechanism for regulating activity, it is likely that activation mechanism(s) depend on stimuli specific to each respective class of PI3K.
The compound PI103 (3-(4-(4-morpholinyl)pyrido[3′,2′:4,5]furo[3,2-d]pyrimidin-2-yl)phenol) inhibits PI3Kα and PI3Kg as well as the mTOR enzymes with IC50 values of 2, 3, and 50-80 nM respectively. I.P. dosing in mice of this compound in human tumor xenograft models of cancer demonstrated activity against a number of human tumor models, including the glioblastoma (PTEN null U87MG), prostate (PC3), breast (MDA-MB-468 and MDA-MB-435) colon carcinoma (HCT 116); and ovarian carcinoma (SKOV3 and IGROV-1); (Raynaud et al, Pharmacologic Characterization of a Potent Inhibitor of Class I Phosphatidylinositide 3-Kinases, Cancer Res. 2007 67: 5840-5850).
The compound ZSTK474 (2-(2-difluoromethylbenzoimidazol-1-yl)-4,6-dimorpholino-1,3,5-triazine) inhibits PI3Kα and PI3Kg but not the mTOR enzymes with an IC50 values of 16, 4.6 and >10,000 nM respectively (Dexin Kong and Takao Yamori, ZSTK474 is an ATP-competitive inhibitor of class I phosphatidylinositol 3 kinase isoforms, Cancer Science, 2007, 98:10 1638-1642). Chronic oral administration of ZSTK474 in mouse human xenograft cancer models, completely inhibited growth which originated from a non-small-cell lung cancer (A549), a prostate cancer (PC-3), and a colon cancer (WiDr) at a dose of 400 mg/kg. (Yaguchi et al, Antitumor Activity of ZSTK474, a New Phosphatidylinositol 3-Kinase Inhibitor, J. Natl. Cancer Inst. 98: 545-556).
The compound NVP-BEZ-235 (2-methyl-2-(4-(3-methyl-2-oxo-8-(quinolin-3-yl)-2,3-dihydro-1H-imidazo[4,5-c]quinolin-1-yl)phenyl)propanenitrile) inhibits both PI3Ka and PI3Kg as well as the mTOR enzymes with IC50 values 4, 5, and “nanomolar”. Testing in human tumor xenograft models of cancer demonstrated activity against human tumor models of prostrate (PC-3) and glioblastoma (U-87) cancer. It entered clinical trials in December of 2006 (Verheijen, J. C. and Zask, A., Phosphatidylinositol 3-kinase (PI3K) inhibitors as anticancer drugs, Drugs Fut. 2007, 32(6): 537-547).
The compound SF-1126 (a prodrug form of LY-294002, which is 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one) is “a pan-PI3K inhibitor”. It is active in preclinical mouse cancer models of prostrate, breast, ovarian, lung, multiple myeloma, and brain cancers. (Verheijen, J. C. and Zask, A., Phosphatidylinositol 3-kinase (PI3K) inhibitors as anticancer drugs, Drugs Fut. 2007, 32(6): 537-547).
Although it seems clear that inhibition of the a isoform is essential for the antitumor activity of PI3K inhibitors, it is not clear whether a more selective inhibitor of a particular PI3K isoform may lead to fewer unwanted biological effects. It has recently been reported that non-PI3Kα class I isoforms (PI3Kβ, δ and γ) have the ability to induce oncogenic transformation of cells, suggesting that nonisoform-specific inhibitors may offer enhanced therapeutic potential over specific inhibitors.
Selectivity versus other related kinases is also an important consideration for the development of PI3K inhibitors. While selective inhibitors may be preferred in order to avoid unwanted side effects, there have been reports that inhibition of multiple targets in the PI3K/Akt pathway (e.g., PI3Kα and mTOR [mammalian target of rapamycin]) may lead to greater efficacy. It is possible that lipid kinase inhibitors may parallel protein kinase inhibitors in that nonselective inhibitors may also be brought forward to the clinic.
Mammalian Target of Rapamycin, mTOR, is a cell-signaling protein that regulates the response of tumor cells to nutrients and growth factors, as well as controlling tumor blood supply through effects on Vascular Endothelial Growth Factor, VEGF. Inhibitors of mTOR starve cancer cells and shrink tumors by inhibiting the effect of mTOR. All mTOR inhibitors bind to the mTOR kinase. This has at least two important effects. First, mTOR is a downstream mediator of the PI3K/Akt pathway. The PI3K/Akt pathway is thought to be over activated in numerous cancers and may account for the widespread response from various cancers to mTOR inhibitors. The over-activation of the upstream pathway would normally cause mTOR kinase to be over activated as well. However, in the presence of mTOR inhibitors, this process is blocked. The blocking effect prevents mTOR from signaling to downstream pathways that control cell growth. Over-activation of the PI3K/Akt kinase pathway is frequently associated with mutations in the PTEN gene, which is common in many cancers and may help predict what tumors will respond to mTOR inhibitors. The second major effect of mTOR inhibition is anti-angiogenesis, via the lowering of VEGF levels.
In lab tests, certain chemotherapy agents were found to be more effective in the presence of mTOR inhibitors. George, J. N., et al., Cancer Research, 61, 1527-1532, 2001. Additional lab results have shown that some rhabdomyosarcoma cells die in the presence of mTOR inhibitors.
There are three mTOR inhibitors, which have progressed into clinical trials. These compounds are Wyeth's Torisel, also known as 42-(3-hydroxy-2-(hydroxymethyl)-rapamycin 2-methylpropanoate, CCI-779 or Temsirolimus; Novartis' Everolimus, also known as 42-O-(2-hydroxyethyl)-rapamycin, or RAD 001; and Ariad's AP23573 also known as 42-(dimethylphopsinoyl)-rapamycin. The FDA has approved Torisel for the treatment of advanced renal cell carcinoma. In addition, Torisel is active in a NOS/SCID xenograft mouse model of acute lymphoblastic leukemia [Teachey et al, Blood, 107(3), 1149-1155, 2006]. On Mar. 30, 2009, the U.S. Food and Drug Administration (FDA) approved Everolimus (AFINITOR™) for the treatment of patients with advanced renal cell carcinoma. AP23573 has been given orphan drug and fast-track status by the FDA for treatment of soft-tissue and bone sarcomas.
The three mTOR inhibitors have non-linear, although reproducible pharmacokinetic profiles. Mean area under the curve (AUC) values for these drugs increase at a less than dose related way. The three compounds are all semi-synthetic derivatives of the natural macrolide antibiotic rapamycin. It would be desirable to find fully synthetic compounds, which inhibit mTOR that are more potent and exhibit improved pharmacokinetic behaviors.